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Thread: compression driver low limit

  1. #1
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    compression driver low limit

    Besides the horn and the suspension of the diaphragm. ( I suppose the space behind the diaphragm could be counted as part of the suspension) What determines the low limit of a compression driver?

    Since I started this thread, and it currently only has one post, I trust no one will post requesting my post be removed as being off topic. I hope I stuck it in an area that is not considered hallowed ground.

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    Hello
    Many things can affect the low limit of compression drivers. The diaphragm material, the surround design, diaphragm size, rated power handling relates to how much xmas before the diaphragm crashes into the phase plug. JBL's 2482 mid range driver uses the same basic driver body as the 2440 and 2441 drivers with just a different diaphragm. The 2482 has a heavy phenolic diaphragm with a large half roll surround to allow for more xmas and is rated down to something like 300hz at full power. The 2482 diaphragm uses a larger mounting ring so you can not interchange other diaphragms on a 2482 body.
    The horn also great effects the low operating limit of the driver that it is bolted to. Generally operating a horn and driver at least one half octave above the published low frequency limit will sound better.

    Mike Caldwell

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    Senior Member Hoerninger's Avatar
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    Quote Originally Posted by Thom View Post
    Since I started this thread, and it currently only has one post, I trust no one will post ... . I hope I stuck it in an area that is not considered hallowed ground.
    Take it easy, not everybody comes around at any time.
    Mike has given a comprehensive answer. It is best to follow the manufacturers advice.

    In its simplest form it can be explained with Thiele-Small-parameters (TSP), they depend on all the mentioned electrical and mechanical parameters. (TSP are absolute unusual for compression drivers.)

    A short discussion of frequency limits of (sufficiently) horn loaded speakers you can find at http://www.jblpro.com/pages/tech_lib.htm
    in paper tn_v1n24.pdf on page 3 (fig.5).
    ___________
    Peter

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    I guess I was wondering why in particular the 2435 with a 3 inch vc isn't supposed to be good below 1khz.

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    Senior Member CONVERGENCE's Avatar
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    I don't know much about JBL drivers but the ALTEC 290 series
    290-8L had a frequency response of 300 to 8000 HZ.
    Of course you need a 300 Hz horn.

    Here are the specs.
    Attached Images Attached Images  

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    Senior Member CONVERGENCE's Avatar
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    If you're interested in brand new drivers Great Plains Audio makes
    the 290 under 390 model.

    The web site is:

    http://www.greatplainsaudio.com/downloads.html

    http://www.greatplainsaudio.com/downloads/390series.pdf

    ...............................................

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    Senior Member Steve Schell's Avatar
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    Diaphragm excursion is the main limiting factor in using compression drivers at lower frequencies. With a constant piston area and power output the excursion quadruples when the frequency is halved. It is also very important that the horn loads the driver well at the low frequency limit to avoid overexcursion; no running 300Hz. at power from a 500Hz. horn!

    Frank Massa of RCA published a very useful equation in the mid 1930s. With it a chart of power output at various excursions and different frequencies can be built:

    P = .0034 X f squared X d squared X Ap squared, all the foregoing divided by Ah

    Where:

    Ap = diaphragm area in square inches
    Ah = throat area in square inches
    f = frequency in Hz.
    d = peak diaphragm amplitude in inches
    P = power output in acoustical watts

    It is assumed that the driver is loaded by an exponential horn of low enough flare constant to load the driver to the lowest frequency of interest. The throat area of a traditional compression driver would be the combined area of the exit paths on the surface of the phasing plug. For a woofer on a flat baffle, the throat area would be that of the hole in the baffle / throat area at the beginning of the horn.

    As can be seen in the equation, Ah exerts a great influence, as everything else is divided by it. Ap divided by Ah is the driver's compression ratio. Most compression drivers have a ratio of about 10:1, which is how high output at low excursion is obtained. One can only go so far with upping the compression ratio, as eventually the motor would not be able to push the load. This is one reason why compression drivers tend to have very strong motors.

    I made charts of the drivers my partner and I have been building to judge their output capabilities. Our Cogent DS-1428 mid driver would produce 1.01 acoustic watts output at 320Hz. with .004" excursion according to the equation. Correspondingly, it would produce 4.04 watts at 640Hz. and 16.18 watts at 1280Hz. with the same excursion, though the latter would require excessive power input. Our DS-1448 bass compression driver has about the same excursion as the 1428 at 320Hz., and would produce 1.11 watts at 80Hz. with .020" excursion.

    I also made a chart for our big horn subwoofer that uses a single 15" woofer. It should produce 2.26 acoustical watts at 20Hz. with .080" excursion. This is well within the .150" limit of the Altec 515-16G driver currently installed.

    Assuming 25% efficiency, which is reasonable for most compression drivers on adequate horns, multiplying the output power by four would reveal the input power needed.

    A tiny trapped air volume behind the diaphragm can limit low frequency output. I once experimented with a pair of Emilar EC-600 drivers (6" diaphragm) on a pair of long 50Hz. horns. Little output below 100Hz. was obtained until I removed the rear covers, then the output was uniform down to about 65Hz.

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    RIP 2011 Zilch's Avatar
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    Quote Originally Posted by Thom View Post
    I guess I was wondering why in particular the 2435 with a 3 inch vc isn't supposed to be good below 1khz.
    800 Hz, actually, at derated power. It's throatless, mating with rapid-flare horns and waveguides:

    http://www.jblpro.com/pub/technote/tn_v1n21.pdf

    What if you load them with slower-flare horn throats?

    A couple of members are trying that.

    Consider also 2450 and its throatless version, 2450SL et. seq.

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    Thanks much. But the more I get the more I want. Is there any significant difference between altecs 1.4 and jbl's 1.5? It would appear to me (probably wrong) that the old horn (providing it's the right size) would be a good match for the new driver but not the other way. I understand that some 2435 have their suspension glued. Is this readily apparent. Can this be remedied? Are there any booby traps inside or tricks in making on play right after dis/ re assy. Can you successfully put a mid and high driver on a two driver horn. Do lower freq horns generally not do so well way up there anyway. I'm not greedy. Even one answer to one question is appreciated.

    Is there a theoretical maximum magnetic intensity? Like there is a max vacuum and therefor a max lift on a pump. Anyone's pump. And there is a max cold. There is a max temp with flame, with arc with plasma etc. Or a max mag intensity with various materials< either as magnets or as cores? I couldn't understand the fascination with field coils but if you can build a stronger magnet with them I understand, some. Everybody still builds their pm drivers with less than the strongest magnet they could.

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    Quote Originally Posted by Thom View Post
    I guess I was wondering why in particular the 2435 with a 3 inch vc isn't supposed to be good below 1khz.
    Why don't you just call the engineer at JBL and flat out ask him? In fact, just reference him to this post and he will respond if he has the time. That way everyone gets the answer without any middlemen or middlewomen involved.

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    Can one actually do that?

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    Quote Originally Posted by Thom View Post
    Can one actually do that?
    Well this would be a pretty useless forum if you couldn't now wouldn't it?

    All you'd end up with then is Lansing Heritage done Quick and Dirty.

    And on that note, I am making a public appeal to anyone who has a whole ton of time on their hands and would love to spend it cranking out seven or eight posts per day (all substance, no filler). Another renaissance man or woman who possesses incredible depth and breadth of knowledge and can offer a different perspective, peferrably someone with outstanding writing skills.

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    Senior Member Steve Schell's Avatar
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    Thom, it is possible to combine drivers of different frequency ranges in one horn, but it must be done strategically. Tom Danley's patented concept for the Unity Horn is a shining example. The high frequency driver is placed at the apex of a conical horn, and the energy from midrange drivers enter the throat somewhat downstream. He has built even three way systems this way. When the physical distances and the crossover are done very carefully, a wide bandwidth device with excellent phase response and freedom from lobing results.

    Yorkville Sound builds a version of the Unity under license:

    http://www.yorkville.com/products.asp?type=29&cat=38

    The maximum gap flux density I am aware of in louspeaker motors is about 24 kilogauss, and this is obtained using permandur pole pieces that are saturated by the magnetic circuit. Permandur is a very special alloy that contains close to 50% cobalt. This is a difficult and expensive way of building a driver, but some like Lowther, ALE and Goto have done it.

    Field coil circuits can be designed to provide most any amount of magnetomotive force, of course the same can be said for permanent magnet circuits. Many (including me) feel that field coils provide listening benefits that have yet to be explained fully. Perhaps it is that the gap flux is more resistant to modulation by voice coil currents. The field coil is connected to a low impedance power supply, where a permanent magnet circuit contains a finite store of energy. Whatever the reason, good field coil drivers tend to have a family sound that is dynamic and detailed in the extreme, yet induces a sense of relaxation in the listener. They are quite addictive.

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    Steve thank you. I appreciate it when you take time to jump in on driver questions. What I had in mind was a standard Altec dual driver throat on a horn that uses a separate throat and I can't really infer from what they are doing that it would or would not work ok. I guess the trick would get something that at least looks good and chrome it if it won't play.

    At first thought it would seem that the action is magnet against (or toward) magnet and a stronger magnet would be like turning up the power. There must be something to that, as the speakers with the stronger gaps are more efficient, but there is more to it than that or these days there wouldn't be much fuss about magnets at all. I know on dc motors (a speaker is a motor or at least part of one) higher armature is faster and more field is more torque and of course if torque is limiting speed then more torque is more speed, and there are volumes published about it that one needn't be a rocket scientist to comprehend. Are there such books on drivers? What I've seen have either been "see Jane turn on the speaker" or something I'd have to go back to school to have a chance of understanding.

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    Quote Originally Posted by Thom View Post
    Steve thank you. I appreciate it when you take time to jump in on driver questions.
    We all do.

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